Other Key Science Projects: Transients, EoR & Cosmic Rays

Philip BestInstitute for Astronomy, Edinburgh

17th June 2009

The Transients KSPPIs: Ralph Wijers, Rob Fender, Ben StappersAim: identify, monitor and study all transient and variable radio phenomena

- Accreting black holes / neutron stars- GRB afterglows- Pulsars- Extrasolar planets- Active flare stars- Counterparts to gravitational wave sources- Serendipity- SETI- ......

LOFAR as a transient monitor

LOFAR’s large collecting area, vast field-of-view and multiple beams give it an unprecedented ability to monitor the sky for transient phenomena

LOFAR Transients ‘Modes’

1. Radio Sky Monitor- Monitoring of a large fraction of the sky ~daily

2. Targetted surveys- e.g. for pulsars, nearby star systems, known active/

interesting systems, monitoring of Virgo cluster, follow-up of newly discovered transients

3. Piggybacking- Search all LOFAR observations with automated transient-

finding tools

Mode 3 will clearly have a large overlap with Surveys

The transient buffer boards

“What was going on 20 seconds ago?”

•Raw data recorded in RAM buffers

•Possible to reform images in any direction on the sky using the RAM data, in response to both internal or external triggers

•Data period that can be stored in RAM buffers depends on bandwidth and number of antennae/beams stored, but is typically a few seconds

•Tool for searching for very rapid, coherent events

Pulsars with LOFAR

• Complete survey of pulsars in the northern sky

• Sources for the pulsar timing array (-> grav. waves)

• Find rare pulsars (e.g. RRAT; BH/NS binary); probe ISM

Extrasolar planets

Scaling Jupiter’s radio emission to account for much stronger stellar winds from hot Jupiters, we could see radio bursts to 10s of parsec

- inclination-independent method of finding new planets?

- provides unique new info, e.g. rotation rate

- requires low frequencies

Epoch of Reionisation KSPPI: Ger de Bruyn, Michiel Brentjens, Leon Koopmans, Saleem ZaroubiAim: To detect the Epoch of Reionisation through the redshifted 21cm hyperfine transition of neutral hydrogen

- The “Epoch of Re-ionisation” occurred when the first astrophysical ionising sources turned on in the Universe.

- Lyman-α photons from these sources decouple the spin temperature of neutral hydrogen from the CMB temperature, resulting in a signal in the 21cm line.

- The EoR is believed to occur at z=7-11, placing the redshifted 21cm line within the LOFAR high band.

LOFAR EoR SignalFor Ts >> TCMB the brightness temperature differential depends only on the overdensity and neutral fraction, so can be reconstructed from simulations.\

In practice, however, there are many complications:- signal very weak requiring very long exposure times- foreground signals much larger, and variable

Challenge: removing foregrounds

From Jelic & Zaroubi (2007)

EoR observing plan

• 5 blank fields with low galactic foreground

• 6-point tile of observations in each field

• Full 48MHz frequency coverage, repeated twice to cover whole of 110-190 MHz range

• About 300 hours on-sky per pointing, using only core and short-baseline stations

• Total requirement of ~150 days with full LOFAR

• Signal ~0.2σ / beam - but statistically detectable

- depth similar to deepest fields of Surveys, but not clear if same fields can be used (EoR need for low galactic background vs Surveys need for multi-wavelength data)

Alternative: 21cm forestIf sufficiently bright radio sources can be found within the EoR, then the EoR can be studied in absorption towards these sources through the 21cm forest.

A major goal of the Surveys KSP is to find such sources.

Figure: EoR absorption features seen towards an 18mJy z=8 radio galaxy by the Square Kilometer Array (from Carilli 2005)

Alternative: 21cm forestIf sufficiently bright radio sources can be found within the EoR, then the EoR can be studied in absorption towards these sources through the 21cm forest.

A major goal of the Surveys KSP is to find such sources.

Figure: true (white) and observed (blue) radio spectrum of a 50mJy radio source at z=7.5, in a 1500 hr (1 beam) LOFAR integration; 21cm absorption visible above 167MHz.

Cosmic Rays KSP

PIs: Heino Falcke, Jörg HörandelAims: to detect and study ultra-high energy particles

- where are ultra-high energy cosmic rays produced?- how are they produced?- what are they made of?- what is the exact shape of their energy distribution?- can high energy neutrinos be detected?

Data can be reconstructed from transient buffer boards. Many observations can piggyback on other telescope uses.

Radio detection of Cosmic Rays

Ultra-high energy cosmic ray (UHECR) produces particle shower in atmosphereElectrons & positrons emit synchrotron radiation which adds coherently at low freq.Can cause GJy flames on tens of nano-second timescalesDepending on cosmic ray energy, detection can be triggered at antenna, station or full array levelTime delays between different antenna give excellent shower front direction, composition & energy measurements

AUGER: UHECR & AGNAUGER collaboration (Science, 2007) found UHECR directions correlated with locations of nearby AGN (probability ~ 2x10-3)

Cosmic Ray energy spectrum

Particle astrophysics & the moon

•High energy neutrinos can produce a coherent radio burst when interacting with the lunar regolith

•LOFAR could detect these, if pointed at the moon

•Probes new energy scales, above LHC energy range

•Any detection would imply new physics / local source

Conclusions

LOFAR will be tackling a very wide range of science goals

Many of the Key Science Projects have considerable scope for sharing observations (piggybacking etc), which will need detailed planning